1. Field of the Invention
The invention relates to a developing unit in a xerographic type image forming apparatus used in a printer, a copying machine, a facsimile device, or the like and, more particularly, to a structure of a developing unit capable of supplying a developer so as to improve the image quality.
2. Description of the Related Art
Heretofore, as a developing unit wherein a two-component developer, composed of a toner and a magnetic carrier, is fed for development to a developing area on the surface of a photosensitive drum, there has been known, for example, a developing unit such as disclosed in Japanese Patent Publication No. Hei 4-48232. According to this conventional developing unit, a developing roller is disposed within a vacant chamber adjacent to the photosensitive drum, the developing roller comprising a developing sleeve adapted to rotate about a horizontal axis and a magnet roller having N and S poles magnetized alternately in the circumferential direction on the inner diameter side of the developing sleeve, the magnet roller being rotated in the direction opposite to the rotating direction of the developing sleeve to form a magnetic brush on the sleeve surface, thereby feeding the developer to the drum surface, while downstream in the toner conveying direction by the developing sleeve in the vacant chamber there is disposed a trimmer blade for restricting the height of the magnetic brush and hence the layer thickness of the developer.
The vacant chamber and a toner supply chamber provided with a toner agitating member are partitioned using upper and lower partition plates. In a stagger portion between the upper and lower partition plate edges is a toner supply slit extending in the axial direction of the developing sleeve. The toner supply slit is formed within an area where the magnetic field induced by the magnet roller extends. Within the vacant chamber, the toner which is supplied through the toner supply slit and a magnetic carrier pre-stored in the vacant chamber are agitated in the axial direction of the developing sleeve by means of fins formed on the inner surface of the upper partition plate, whereby an initial developer (initial toner, which is a mixture of magnetic carrier and toner), fed beforehand into the vacant chamber, adheres to the outer peripheral surface of the developing sleeve to form a magnetic brush of a predetermined height on the sleeve. For the formation of the image, toner is supplied from the toner supply chamber side.
In the above construction, however, when the toner to be supplied has magnetism and the partition plates are formed of a non-magnetic material that easily conducts magnetism (less magnetism shielding property), such as a synthetic resin, and when the magnetic toner in the toner supply chamber is fed into the vacant chamber through the toner supply slit by means of the toner agitating member, the action of magnetism from the magnet roller reaches the magnetic toner on the opposite side (the toner supply chamber side) through the partition plate wall portion so that the magnetic toner is attracted magnetically to, for example, the wide surface of the partition plate on the toner supply chamber side and is held there. As a result, the clearance of the toner supply slit between the upper and lower partition plates is narrowed by the magnetic toner attracted and held as described above. Consequently, it becomes difficult to convey the toner to the vacant chamber side, hence deteriorating the effect of replenishing the developing roller with a predetermined amount of toner. Further, there has been another problem such that the toner deposited on the inner surface of each partition plate solidifies due to being held over a long period.
Additionally, by only forming fins on the inner surface of the upper partition plate, the frictional charging action induced by the agitation of toner and carrier in the vacant chamber has been incomplete.